Manufacturing processes of semiconductor devices that require comparatively thick crystalline film, like power devices such as insulated gate bipolar transistors (IGBTs), have conventionally employed the epitaxial growth technique to form a single-crystal thin film on a substrate, such as a semiconductor wafer, by vapor-phase growth. In a film deposition apparatus used in the epitaxial growth technique, a substrate such as a wafer is placed within a deposition chamber maintained at atmospheric or reduced pressure. The placed substrate is heated while the raw material gas for film deposition (raw material gas) is supplied to the deposition chamber. This causes chemical reaction such as pyrolysis reaction or hydrogen reduction reaction of the raw material gas at the surface of the substrate, so that an epitaxial film is formed on the substrate.
The film deposition apparatus uses a curvature measurement apparatus (a warpage measurement apparatus) which measures the curvature of the substrate as a measurement object. The curvature measurement apparatus has been used mainly for optimizing the process procedure. In recent years, however, mass-production apparatuses have become required to perform constant monitoring of warpage by using the curvature measurement apparatus. In a process of forming gallium nitride (GaN) film on an 8-inch silicon wafer, for example, it is very important to monitor how much the wafer warps during the deposition process because silicon and GaN thin film have a difference in coefficient of thermal expansion and a large mismatch of crystal lattice constants, and moreover because the process employs film deposition conditions in which the temperature is repeatedly raised and lowered in a wide range. If the monitoring of warpage is neglected, the product quality of wafers can be degraded because rupture of wafers, small cracks in the deposited thin film, or the like may occur during the deposition or when the temperature is lowered after the deposition. Accordingly, the monitoring of warpage is necessary for optimization of the process procedure prior to mass production but is also necessary to preserve the product quality even in the mass production in which the conditions of the deposition chamber gradually change.
A current prevailing curvature measurement apparatus causes two or more laser beams to travel side by side and enter a substrate through a window of the deposition chamber, detects the positions of at least two laser beams which are reflected from the substrate and return through the aforementioned window, and reads the distance between the detected positions. Here, the measurement method using three or more laser beams is not different in principle from the measurement method using two laser beams. For the sake of simplification, the measurement method using only two laser beams is explained below. As the way of detecting two laser beams, generally employed is a CCD (charge-coupled device) two-point detection method that simultaneously detects the two laser beams with a single two-dimensional CCD. In this detection method, the two laser beams are incident on a same CCD surface (a light receiving surface) and the positions of the two laser beams on the CCD detection plane are obtained as two points in the pixels of the CCD. The distance between the detected points is calculated by image processing and is converted into a curvature.
However, in the aforementioned CCD two-point detection method, when the substrate significantly warps, the detected points sometimes coincide with each other and have no distance therebetween. In this case, although the substrate has a certain curvature, the curvature cannot be measured. Moreover, even if the detected points do not coincide with each other, the S/N ratio is lowered because of the resolution limitation of the CCD when the substrate warps significantly. Furthermore, when the distance between the detected points needs to be reduced for the purpose of reducing the size of the window through which the two laser beams pass, the conditions become more sever, and the S/N ratio is lowered.